We work with scheduling software. One of the schedule rules established in our software is "Hours of Availability" (the hours someone works). If we were to send an e-mail to inform someone that their hours changed, would be say "Your Hours of Availability were changed" or "Your Hours of Availability was changed"...? For the record, my vote is for were changed, since Hours is plural and of Availability is a prepositional phrase. The other standpoint is that "Hours of Availability" is a singular proper noun, thus was would be the correct verb.

I will offer an opinion, but you should wait for others, since I do not consider myself an expert. The question seems to have a few ambiguous aspects, which might lead to differences of opinion.

You say "Hours of Availability" is [the title?] of software. Then-- "Hours of Availability" was changed might mean that there are changes in the software (though not necessarily in the hours). However, if your Hours of Availability', (with capital letters), is an accurate writng of the phrase in question, we might assume that it is like saying your software program entitled 'Hours of Availability' was changed. If what you are changing is the hours [of availability], then I would vote for your hours of availabilitywere changed.

It would be helpful to know just exactly what is being changed: the particular hours [when the worker is available], or general program features of Hours of Availability.

In your paragraph, I believe, 'Scheduling System' is the software name, and, 'Hours of Availability' is one of the rules defined in the software. Does each user have a different 'Hours of Availability'?